In at least one known CT system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile of the object.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object.
One method for reconstructing an image from a set of projection data is referred to in the art as the filtered backprojection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
Detector elements are configured to perform optimally when impinged by x-rays travelling a straight path from the x-ray source to the detector elements. Particularly, detector elements typically include scintillation crystals which generate light events when impinged by an x-ray beam. These light events are output from each detector element and directed to photoelectrically responsive materials in order to produce an electrical signal representative of the attenuated beam radiation received at the detector element. Typically, the light events are output to photomultipliers or photodiodes which produce individual analog outputs. Detector elements thus output a strong signal in response to impact by a straight path x-ray beam.
X-rays often scatter when passing through the object being imaged. Particularly, the object often causes some, but not all, x-rays to deviate from the straight path between the x-ray source and the detector. Therefore, detector elements are often impinged by x-ray beams at varying angles.
System performance is degraded when detector elements are impinged by these scattered x-rays. When a detector element is subjected to multiple x-rays at varying angles, the scintillation crystal generates multiple light events. The light events corresponding to the scattered x-rays generate noise in the scintillation crystal output, and thus cause artifacts in the resulting image of the object.
To reduce the effects of scattered x-rays, scatter collimators are often disposed between the object of interest and the detector array. Such collimators are constructed of x-ray absorbent material and positioned so that scattered x-rays are substantially absorbed before impinging upon the detector array. One known scatter collimator is described, for example, in U.S. Pat. No. 5,293,417, assigned to the present assignee. It is important for a scatter collimator to be properly aligned with both the x-ray source and the detector elements so that substantially only straight path x-rays impinge on the detector elements. It is also important that a scatter collimator shield radiation damage sensitive detector elements from x-rays at certain locations, such as the detector element edges.
Known collimators are complicated and cumbersome to construct. In addition, it is difficult to satisfactorily align known collimators with the x-ray source and the detector elements to both absorb scattered x-rays and shield sensitive portions of the detector elements.
Even when a scatter collimator is sufficiently aligned and positioned, detector elements may still generate artifacts. Particularly, detector elements are known to exhibit output gain loss after being subjected to accumulated exposure to x-ray dosage. The extent of output gain loss is directly related to the accuracy and usefulness of the detector element. After exhibiting excessive output gain loss, the detector element must be replaced. Replacing individual detector elements, as well as entire detector arrays, is a time consuming and cumbersome process.
To reduce output gain loss, and thus extend the operational life of detector elements, detector arrays typically include reflectors. Particularly, detectors typically include detector elements forming either one-dimensional or two-dimensional arrays of scintillation crystals having interstitial reflectors. As explained above, when impinged by an x-ray beam, the scintillation crystals produce light events. The reflectors are used to prevent the light within each crystal from escaping the crystal, i.e., to eliminate output gain loss. The interstitial reflectors typically are constructed of foils, coatings or other cast-in-place reflective materials. However, the reflective materials used for the reflectors typically include organic materials which exhibit radiation damage effects over time. Such radiation damage reduces reflector reflectivity, which results in output gain loss. Accordingly, it is desirable to shield the reflectors with the scatter collimator.
It would be desirable to provide a scatter collimator that is not complicated and cumbersome to construct, and that effectively absorbs scattered x-rays and substantially prevents such x-rays from impinging the detector array. It also would be desirable to further reduce detector element output gain loss without significantly increasing the costs of detector elements and detector arrays.